Heterocyclic-methylthioalkyl-guanidines

ABSTRACT

The compounds are N-(heterocyclomethylthioalkyl)derivatives of alkylguanidines, C-alkyl and C-aryl amidines and S-alkylisothioureas. The compounds may also have N&#39;-lower alkyl or N&#39;-(heterocyclomethylthioalkyl)substituents. Three compounds of the invention are S-methyl-N-[2-(5-methyl-4-imidazolylmethylthio)ethyl]isothiourea, N,N&#39;-bis-[2-(5-methyl-4-imidazolylmethylthio)ethyl]acetamidine, N,N&#39;-bis-[2-(5-methyl-4-imidazolylmethylthio)ethyl]N&#34;-methylguanidine. The compounds of this invention are histamine H 2  -antagonists.

This is a division of application Ser. No. 794,178 filed May 5, 1977, U.S. Pat. No. 4,107,319 which is a division of application Ser. No. 551,220 filed Feb. 19, 1975, now U.S. Pat. No. 4,036,971.

This invention relates to pharmacologically active compounds, to pharmaceutical compositions containing these compounds and to methods of blocking histamine H₂ -receptors by administering these compounds. The compounds of the invention can exist as acid addition salts but, for convenience, reference will be made throughout this specification to the parent compound.

Many physiologically active substances elicit their biological actions by interaction with specific sites known as receptors. Histamine is such a substance and has a number of biological actions. Those biological actions of histamine which are inhibited by drugs commonly called "antihistamines" of which mepyramine is a typical example, and diphenhydramine and chlorpheniramine are other examples, are mediated through histamine H₁ -receptors (Ash and Schild, Brit. J. Pharmac. Chemother, 27, 427, (1966)). However, other of the biological actions of histamine are not inhibited by "antihistamines" and actions of this type which are inhibited by a compound described by Black et al. (Nature, 236, 385 (1972)) and called burimamide are mediated through receptors which are defined by Black et al. as histamine H₂ -receptors. Thus histamine H₂ -receptors may be defined as those histamine receptors which are not blocked by mepyramine but are blocked by burimamide. Compounds which block histamine H₂ -receptors are referred to as histamine H₂ -antagonists.

Blockade of histamine H₂ -receptors is of utility in inhibiting the biological actions of histamine which are not inhibited by "antihistamines". Histamine H₂ -antagonists are therefore useful, for example, as inhibitors of gastric acid secretion, as anti-inflammatory agents and as agents which act on the cardiovascular system, for example as inhibitors of the effects of histamine on blood pressure; in the treatment of certain conditions, for example inflammation and in inhibiting the actions of histamine on blood pressure, a combination of histamine H₁ - and H₂ -antagonists is useful. The compounds of this invention are histamine H₂ -antagonists. These compounds are represented by the following formula: ##STR1## wherein R₁ represents a grouping of the structure shown in Formula II:

    het--CH.sub.2 --S--(CH.sub.2).sub.n --                     FORMULA II

wherein Het is a nitrogen containing 5 or 6 membered heterocyclic ring selected from imidazole, pyridine, thiazole, isothiazole, thiadiazole, isoxazole and triazole which is optionally substituted by lower alkyl, hydroxyl, halogen or amino: n is 2 or 3: R₂ is lower alkyl, phenyl optionally substituted by hydroxy or mercapto, SR₄ or, when R₃ is other than hydrogen, NHR₅ : R₃ is hydrogen, lower alkyl or R₁ : R₄ and R₅ which may be the same or different are lower alkyl: and R₃ may be linked with R₄ or R₅ to form an additional 5-membered ring such as thiazoline or imidazoline, or a pharmaceutically acceptable acid addition salt.

Throughout the present specification and claims, by the term "lower alkyl" we mean an alkyl group containing from 1 to 4 carbon atoms, preferably methyl.

It will be understood that the structure illustrated in Formula I is only one of several representations and that other tautomeric forms are also covered by the present invention.

In a preferred group of compounds n is 2 and it is particularly preferably that Het is imidazole, optionally substituted by methyl or halogen; thiazole; or isothiazole or pyridine optionally substituted by methyl, hydroxyl or halogen. Halogen is preferably chloro or bromo.

Useful series of compounds are (a) that wherein R₃ is identical to R₁, (b) that wherein R₂ is lower alkylamino and (c) that wherein R₂ is lower alkylthio. Examples of specific compounds falling within the scope of the present invention are:

S-methyl-N-[2-(5-methyl-4-imidazolylmethylthio)ethyl]-isothiourea

N,n'-bis-[2-(5-methyl-4-imidazolylmethylthio)ethyl]acetamidine

N,n'-bis-[2-(5-methyl-4-imidazolylmethylthio)ethyl]N"-methylguanidine.

The compounds of Formula I wherein R₂ is SR₄ and R₄ does not form an additional 5-membered ring may be prepared by alkylation of a thiourea of Formula III: ##STR2## wherein R₁ and R₃ are as defined in Formula I to give an isothiourea of Formula IV ##STR3## wherein R₄ is lower alkyl. This reaction may be carried out conveniently using hydrogen chloride and the alcohol R₄ OH, preferably at an elevated temperature near to the boiling point of the alcohol R₄ OH. Alternatively the thiourea of Formula III may be alkylated using an alkyl halide, such as methyl iodide, or an alkylsulphate such as diethyl sulphate.

The compounds of Formula I other than those wherein R₂ is lower alkylthio may be prepared by treatment of a compound of Formula V ##STR4## wherein R₃ is as defined in Formula I, R₆ is R₂ or NHR₁, R₁ and R₂ being as defined in Formula I except that R₄ when present is linked to R₃ to form a 5-membered ring such as a thiazoline ring, A is lower alkyl and X is oxygen when R₆ is lower alkyl or phenyl optionally substituted by hydroxy or mercapto, and X is sulphur when R₆ is SR₄, NHR₅, or NHR₁, with an amine of formula R₇ NH₂, R₇ being R₁ when R₆ is R₂, and R₇ being lower alkyl when R₆ is R₁ NH and R₃ is other than hydrogen.

Examples of particular processes falling within the above general process are:

(a) The preparation of compounds of Formula VI ##STR5## wherein R₁, is as defined in Formula I, R₃ is lower alkyl or R₁ in which case the groups R₁ may be the same or different, and R₅ is lower alkyl and R₃ may be linked to R₅ to form a 5-membered ring, by treatment of an isothiourea of Formula IV wherein R₃ is R₁ with either a lower alkylamine or an amine of formula R₁ NH₂.

(b) The preparation of compounds of Formula I, wherein R₂ is SR₄ and R₃ and R₄ are linked to form a 5-membered ring, by the reaction of R₁ NH₂ with an S-lower alkylthiazoline of formula VII: ##STR6## wherein A represents a lower alkyl group.

(c) The preparation of compounds of Formula I wherein R₂ is lower alkyl or phenyl optionally substituted by hydroxy or mercapto by reaction of an amine of formula R₁ NH₂ with an iminoether of Formula VII: ##STR7## wherein A represents a lower alkyl group.

The iminoethers of Formula VIII wherein R₃ is hydrogen may be prepared by the reaction between a nitrile of formula R₂ CN and a lower alkanol AOH. The iminoethers of Formula VIII wherein R₃ is R₁ are generated by the reaction of an amine R₁ NH₂ with an orthoester of formula R₂ --C--(OA)₃.

Alternatively the compounds of Formula I wherein R₂ is mercaptophenyl may be prepared by treating the corresponding hydroxybenzamidine with phosphorus pentasulphide in a solvent such as pyridine.

The compounds of Formula I block histamine H₂ -receptors, that is they inhibit the biological actions of histamine which are not inhibited by "antihistamines" such as mepyramine but are inhibited by burimamide. For example, the compounds of this invention have been found to inhibit histamine-stimulated secretion of gastric acid from the lumen-perfused stomachs of rats anaesthetized with urethane, at doses of from 0.5 to 256 micromoles per kilogram intravenously. This procedure is referred to in the above mentioned paper of Ash and Schild. The activity of these compounds as histamine H₂ -antagonists is also demonstrated by their ability to inhibit other actions of histamine which, according to the above mentioned paper of Ash and Schild, are not mediated by histamine H₁ -receptors. For example, they inhibit the actions of histamine on the isolated guinea pig atrium and isolated rat uterus.

The compounds of this invention inhibit the basal secretion of gastric acid and also that stimulated by pentagastrin or by food.

In addition, the compounds of this invention show anti-inflammatory activity in conventional tests such as the rat paw oedema test, where the oedema is induced by an irritant, the rat paw volume is reduced by subcutaneous injection of doses of a compound of Formula I. In a conventional test, such as the measurement of blood pressure in the anaesthetised cat, the action of the compounds of this invention in inhibiting the vasodilator action of histamine can also be demonstrated. The level of activity of the compounds of this invention is illustrated by the effective dose producing 50% inhibition of gastric acid secretion in the anaesthetized rat of the dose producing 50% inhibition of histamine-induced tachycardia in the isolated guinea pig atrium.

For therapeutic use, the pharmacologically active compounds of the present invention will normally be administered as a pharmaceutical composition comprising as the or an essential active ingredient at least one such compound in the basic form or in the form of an addition salt with a pharmaceutically acceptable acid and in association with a pharmaceutical carrier therefor. Such addition salts include those with hydrochloric, hydrobromic, hydriodic, sulphuric and maleic acids and may conveniently be formed from the corresponding bases of Formula I by standard procedures, for example by treating the base with an acid in a lower alkanol or by the use of the ion exchange resins to form the required salt either directly from the base or from a different addition salt.

Pharmaceutical compositions comprising a pharmaceutical carrier and a compound of Formula I or a pharmaceutically acceptable acid addition salt thereof and methods of blocking histamine H₂ -receptors which comprise administering a compound of Formula I or a pharmaceutically acceptable acid addition salt thereof are also objects of this invention. The pharmaceutical carrier employed may be, for example, either a solid or liquid. Exemplary of solid carriers are lactose, terra alba, sucrose, talc, gelatin agar, pectin, acacia, magnesium stearate, stearic acid and the like. Exemplary of liquid carriers are syrup, peanut oil, olive oil, water and the like.

A wide variety of pharmaceutical forms can be employed. Thus, if a solid carrier is used, the preparation can be tableted, placed in a hard gelatin capsule in powder or pellet form, or in the form of a troche or lozenge. The amount of solid carrier will vary widely but preferably will be from about 25 mg to about 1 g. If a liquid carrier is used, the preparation may be in the form of a syrup, emulsion, soft gelatin capsule, sterile injectable liquid contained for example in an ampoule, or an aqueous or nonaqueous liquid suspension.

The pharmaceutical compositions are prepared by conventional techniques involving procedures such as mixing, granulating and compressing or dissolving the ingredients as appropriate to the desired preparation.

The active ingredient will be present in the compositions in an effective amount to block histamine H₂ -receptors. The route of administration may be oral or parenteral.

Preferably, each dosage unit will contain the active ingredient in an amount of from about 50 mg to about 250 mg.

The active ingredient will preferably be administered one to six times per day. The daily dosage regiment will preferably be from about 150 mg to about 1500 mg.

Advantageously the composition will be made up in a dosage form appropriate to the desired mode of administration, for example as a tablet, capsule, injectable solution or as a cream or ointment for topical application.

The invention is illustrated but in no way limited by the following examples wherein all temperatures are given in degrees Centigrade:

EXAMPLE 1 S-Methyl-N-[2-((5-methyl-4-imidazolyl)methylthio)ethyl]isothiourea hydriodide

A mixture of N-[2-((5-methyl-4-imidazolyl)methylthio)ethyl]thiourea (2.29 g) and methyl iodide (1.56 g) in acetone (45 ml) containing methanol (5 ml) was left at room temperature for 18 hours. Concentration followed by recrystallisation from isopropyl alcohol-petroleum ether (b.p. 60°/80°) gave the title compound (2.3 g) m.p. 128°-131°.

(Found: C, 28.8; H, 4.5; N, 14.8; S, 17.5; I, 33.8; C₉ H₁₆ N₄ S₂ requires: C, 29.0; H, 4.6; N, 15.1; S, 17.2; I, 34.1%).

EXAMPLE 2 N,S-Dimethyl-N'-[2-((5-methyl-4-imidazolyl)methylthio)ethyl]isothiourea dihydriodide

N-methyl-N'-[2-((5-methyl-4-imidazolyl)methylthio)ethyl]thiourea (10.0 g) was dissolved in methanol (400 ml) and 55% aqueous hydriodic acid (12.2 ml) was added, followed by methyl iodide (5.3 ml). The solution was heated under reflux for 4 hours and concentrated. The residue was dissolved in methanol (100 ml) and ether was added to afford the title product as the dihydriodide (5.48 g) m.p. 185°-186° (from isopropyl alcohol).

(Found: C, 23.4; H, 3.9; N, 10.9; I, 49.6; S, 12.6; C₁₀ H₁₈ N₄ S₂.2HI requires: C, 23.4; H, 3.9; N, 10.9; I, 49.4; S, 12.5%).

EXAMPLE 3 2-[2-(4-Imidazolylmethylthio)ethyl]amino-2-thiazoline hydriodide

A solution of 4(5)-[(2-aminoethyl)thiomethyl]imidazole (from the dihydrobromide (5.0 g) and excess potassium carbonate) and 2-methylthio-2-thiazoline hydriodide (4.09 g) in ethanol (100 ml) was heated under reflux for 21 hours. Following concentration the residue was crystallized from methanol-ether and then recrystallised from ethanol to give the title compound (2.9 g) m.p. 148.5°-150.5°.

(Found: C, 29.0; H, 4.0; N, 15.0; S, 17.5; I, 34.0; C₉ H₁₄ N₄ S₂.HI requires: C, 29.2; H, 4.1; N, 15.1; S, 17.5; I, 34.3%).

EXAMPLE 4 N-[2-(5-Methyl-4-imidazolylmethylthio)ethyl]benzamidine dihydrochloride

A solution of ethyl benzimidate hydrochloride (10.84 g) in water (25 ml) was basified with potassium carbonate (8.07 g) and extracted with diethyl ether (3×30 ml). The ethereal extracts were combined and dried over anhydrous potassium carbonate. The potassium carbonate was removed by filtration and the filtrate concentrated to about 30 ml. To the latter was added a solution of 4-methyl-5-[(2-aminoethyl)-thiomethyl]imidazole (5.00 g) in ethanol. The combined solutions were allowed to stand at room temperature for 6 days. The solution was concentrated to give an oil, cooled and acidified with ethanolic hydrogen chloride. The solution was concentrated and the residue recrystallised from isopropanol (with a trace of ether) to give N-[2-(5-methyl-4-imidazolylmethylthio)ethyl]benzamidine dihydrochloride (7.64 g) m.p. 221°-222.5° (from ethanol-ether).

(Found: C, 48.0; H, 5.9; N, 15.8; S, 9.1; C₁₄ H₁₈ N₄ S.2HCl requires: C, 48.4; H, 5.8; N, 16.1; S, 9.2%).

EXAMPLE 5 N-[2-(5-Methyl-4-imidazolylmethylthio)ethyl]-4-hydroxybenzamidine dihydrochloride.

4-Cyanophenol (11.91 g) was dissolved in dry diethyl ether (200 ml). Ethanol (10 ml) was added and the solution saturated with dry hydrogen chloride at 10°. The solution was stored at 0° over weekend. Yellow crystals of ethyl 4-hydroxybenzimidate hydrochloride (4.425 g) deposited. A solution of ethyl 4-hydroxybenzimidate hydrochloride (4.425 g) in water (10 ml) was basified with potassium carbonate (10.0 g) and extracted with diethyl ether (3×30 ml). This solution was dried over anhydrous potassium carbonate and added to a solution of 4-methyl-5-[(2-aminoethyl)thiomethyl]imidazole in dry ethanol (100 ml). Over 7 days at room temperature a crystalline solid was deposited. The crude base was recrystallised from ethanol-ether to give N-[2-(5-methyl-4-imidazolylmethylthio)ethyl]-4-hydroxybenzamidine as the ethanolate (2.12 g) m.p. 136°-138°. A sample of the ethanolate (1.58 g) was converted to the dihydrochloride by the addition of ethanolic hydrogen chloride. The solution was concentrated and the resulting solid recrystallised to give N-[2-(5-methyl-4-imidazolylmethylthio)ethyl]-4-hydroxybenzamidine hydrochloride (1.28 g) (225.5°-228°).

(Found: C, 46.3; H, 5.6; N, 15.2; S, 8.8; Cl, 19.4; C₁₄ H₁₈ N₄ OS.2 HCl requires: C, 46.3; H, 5.6; N, 15.4; S, 8.8; Cl, 19.5%).

EXAMPLE 6 N,N'-bis-[2-(5-Methyl-4-imidazolylmethylthio)ethyl]acetamidine trihydrochloride

A mixture of 4-methyl-5-[(2-aminoethyl)thiomethyl]imidazole (3.00 g), triethyl orthoacetate (1.42 g) and acetic acid (0.525 g) was refluxed for 60 minutes. The reaction mixture was allowed to cool, concentrated and dissolved in water. The aqueous solution was added to a solution of picric acid in boiling isopropanol. On cooling the crystalline picrate formed. This was recrystallised from isopropanol-water (3:1, 400 ml) to give N,N'-bis-[2-(5-methyl-4-imidazolylmethylthio)ethyl]acetamidine tripicrate (5.95 g) m.p. 205°-206°. The tripicrate was converted to the trihydrochloride by the addition of 5 N hydrochloric acid (50 ml) followed by extraction with toluene (5×30 ml). The aqueous solution was concentrated and the residual solid was boiled with isopropanol and gradually crystallised. The crude trihydrochloride was recrystallised from ethanol-ethyl acetate to give N,N'-bis-[2-(5-methyl-4-imidazolylmethylthio)ethyl]acetamidine trihydrochloride (1.89 g) m.p. 230°-232°.

(Found: C, 40.4; H, 6.2; N, 17.5; S, 13.2; Cl, 21.9. C₁₆ H₂₆ N₆ S₂. 3 HCl requires: C, 40.4; H, 6.1; N, 17.7; S, 13.5; Cl, 22.4%).

In a similar manner

(a) N,N'-bis-[2-(3-bromo-2-pyridylmethylthio)ethyl]acetamidine may be prepared from 3-bromo-2-[(2-aminoethyl)thiomethyl]pyridine.

(b) N,N'-bis-[2-(2-thiazolylmethylthio)ethyl]acetamidine may be prepared from 2-[(2-aminoethyl)thiomethyl]thiazole.

(c) N,N'-bis-[2-(3-isothiazolylmethylthio)ethyl]acetamidine may be prepared from 3[(2-aminoethyl)thiomethyl]isothiazole.

(d) N,N'-bis-[2-(3-chloro-2-pyridylmethylthio)ethyl]acetamidine, may be prepared from 3-chloro-2-[(2-aminoethyl)thiomethyl]pyridine.

EXAMPLE 7 2-[2-((5-Methyl-4-imidazolyl)methylthio)ethyl]amino-2-imidazoline dihydrochloride

A solution of 4-methyl-5-((2-aminoethyl)thiomethyl)imidazole (8.55 g) and 2-methylmercapto-2-imidazoline hydriodide (13.2 g) in ethanol (50 ml) was heated under reflux for 15 hours. The residue following concentration was dissolved in ethanol and treated with sodium picrate in hot ethanol. The picrate obtained on cooling was recrystallised from ethanol (16.5 g, m.p. 190°-192°) and treated with hydrochloric acid. Picric acid was removed by toluene extraction and the aqueous layer was concentrated and the residue recrystallised twice from isopropyl alcohol to give the title compound as the dihydrochloride (7.0 g) m.p. 189°-191°.

(Found: C, 38.6; H, 6.3; N, 22.3; S, 10.5; Cl, 22.6. C₁₀ H₁₇ N₅ S.2HCl requires: C, 38.5; H, 6.1; N, 22.4; S, 10.3; Cl, 22.7%).

EXAMPLE 8 N,N'-Dimethyl-N"-[2-(5-methyl-4-imidazolylmethylthio)ethyl]guanidine dihydrochloride

N,N'S-Trimethylisothiouronium iodide (12.0 g) was converted into the sulphate by ion exchange on a resin on the SO₄.sup.═ form (IRA 401). The concentrated eluate of the sulphate was reacted directly with 4-methyl-5-((2-aminoethyl)thiomethyl)imidazole (6.85 g) at 70°-80° for 48 hours. Following concentration, the crude product was converted to a picrate (8 g, m.p. 110°-120° (from aqueous ethanol)). Following conversion to the hydrochloride and purification on an ion-exchange column (OH⁻ form) with elution by hydrochloric acid, the title compound, m.p. 260° (dec) was obtained.

EXAMPLE 9 S-Methyl-N,N'-bis-[2-((5-methyl-4-imidazolyl)methylthio)ethyl]isothiourea trihydrochloride

(i) A solution of 4-methyl-5-((2-aminoethyl)thiomethyl) imidazole (34.0 g) and carbon disulphide (7.6 g) in ethanol (250 ml) was heated under reflux for 6 hours. Concentration followed by chromatographic purification of the product on a column of silica gel with elution by isopropyl alcohol-ethyl acetate followed by isopropyl alcohol-ethanol gave N,N'-bis-[2-(5-methyl-4-imidazolyl)methylthio)ethyl]thiourea (18 g), m.p. 133°-135°.

(Found: C, 47.0; H, 6.1; N, 22.0. C₁₅ H₂₄ N₆ S₃ requires: C, 46.8; H, 6.3; N, 21.9%).

(ii) Dry hydrogen chloride was passed into a solution of N,N'-bis-[2-((5-methyl-4-imidazolyl)methylthio)ethyl]thiourea (7.7 g) in methanol at 80° for 5 hours. Concentration and re-evaporation with isopropyl alcohol afforded S-methyl-N,N'-bis-[2-((5-methyl-4-imidazolyl)methylthio)ethyl]isothiourea trihydrochloride (9.0 g), m.p. 212°-215° (isopropyl alcohol).

(Found: C, 37.6; H, 5.7; N, 16.3; S, 18.6; Cl, 20.5 C₁₆ H₂₆ N₆ S₃. 3 HCl requires: C, 37.8; H, 5.8; N, 16.5; S, 18.9; Cl, 20.9%).

EXAMPLE 10 N,N'-bis-[2-((5-Methyl-4-imidazolyl)methylthio)ethyl]-N"-methylguanidine

A solution of N,N'-bis-[2-((5-methyl-4-imidazolyl)methylthio)ethyl]-S-methylisothiourea trihydrochloride (2.0 g) in 33% ethanolic methylamine (60 ml) was heated under reflux for 3 hours. The mixture was basified with sodium methoxide, filtered and the filtrate was concentrated under reduced pressure. Treatment of the residue with ethanolic picric acid afforded N,N'-bis-[2-((5-methyl-4-imidazolyl)methylthio)ethyl]-N"-methylguanidine tripicrate (2.8 g) m.p. 169°-170°.

(Found: C, 38.0; H, 3.4; N, 20.7; S, 6.0. C₁₆ H₂₇ N₇ S₂.3 C₆ H₃ N₃ O₇ requires: C, 38.2; H, 3.4; N, 21.0; S, 6.0%).

Treatment of the tripicrate with hydrochloric acid and removal of picric acid by extraction with toluene afforded the trihydrochloride.

In a similar manner N,N'-bis-[2-((5-methyl-4-imidazolyl)methylthio)ethyl]-N"-butylguanidine may be prepared using butylamine in ethanol in place of ethanolic methylamine.

EXAMPLE 11 N-[2-(5-Methyl-4-imidazolylmethylthio)ethyl]-4-mercaptobenzamidine

(a) 4-Cyanothiophenol was treated with hydrogen chloride in ether at 0° and the hydrochloride salt produced was treated with potassium carbonate to give ethyl 4-mercaptobenzimidate. Ethyl 4-mercaptobenzimidate was treated with 4-methyl-5-[(2-aminoethyl)thiomethyl]imidazole in dry ethanol at room temperature to give the title product.

(b) N-[2-(5-Methyl-4-imidazolylmethylthio)ethyl]-4-hydroxybenzamidine and a slight excess of phosphorus pentasulphide were refluxed in pyridine for 45 minutes to give the title compound.

EXAMPLE 12 N-[2-(5-Methyl-4-imidazolylmethylthio)ethyl]acetamidine

A mixture of ethyl acetimidate and 4-methyl-5-[(2-aminoethyl)thiomethyl]imidazole in ethanol was allowed to stand at room temperature for 6 days. The mixture was concentrated, treated with ethanolic hydrogen chloride and evaporated to give the title product as the dihydrochloride.

EXAMPLE 13

Treatment of the following thioureas:

a. N-[2-(2-Pyridylmethylthio)ethyl]-N'-[2-((5-methyl-4-imidazolyl)methylthio)ethyl]thiourea.

b. N-[2-(3-Pyridylmethylthio)ethyl]-N'-[2-((5-methyl-4-imidazolyl)methylthio)ethyl]thiourea.

c. N,N'-bis[2-((3-hydroxy-2-pyridyl)methylthio)ethyl]thiourea.

d. N,N'-bis-[2-(3-isoxazolylmethylthio)ethyl]thiourea.

with hydrogen chloride in methanol according to the general procedure of Example 9 (ii) results in the formation of the S-methylisothioureas:

a. S-Methyl-N-[2-(2-pyridylmethylthio)ethyl]-N'-[2-((5-methyl-4-imidazolyl)methylthio)ethyl]isothiourea.

b. S-Methyl-N-[2-(3-pyridylmethylthio)ethyl]-N'-[2-((5-methyl-4-imidazolyl)methylthio)ethyl]isothiourea.

c. S-Methyl-N,N'-bis-[2-((3-hydroxy-2-pyridyl)methylthio)ethyl]isothiourea.

d. S-Methyl-N,N'-bis-[2-((3-isoxazolylmethylthio)ethyl]isothiourea.

and treatment of these S-methylisothioureas with methylamine according to the general procedure of Example 10 results in the formation of the N"-methylguanidines:

a. N-[2-(2-pyridylmethylthio)ethyl]-N'-[2-((5-methyl-4-imidazolyl)methylthio)ethyl]-N"-methylguanidine.

b. N-[2-(3-pyridylmethylthio)ethyl]-N'-[2-((5-methyl-4-imidazolyl)methylthio)ethyl]-N"-methylguanidine.

c. N,N'-bis-[2-((3-hydroxy-2-pyridyl)methylthio)ethyl]N"-methylguanidine.

d. N,N'-bis-[2-(3-isoxazolyl-methylthio)ethyl]N"-methylguanidine.

EXAMPLE 14

Substitution of:

(a) 4-[(2-aminoethyl)thiomethyl]imidazole.

(b) 4-bromo-5-[(2-aminoethyl)thiomethyl]imidazole.

(c) 1-methyl-2-[(2-aminoethyl)thiomethyl]imidazole.

(d) 2-[(2-aminoethyl)thiomethyl]imidazole.

(e) 3-chloro-2-[(2-aminoethyl)thiomethyl]pyridine.

(f) 3-bromo-2-[(2-aminoethyl)thiomethyl]pyridine.

(g) 3-methyl-2-[(2-aminoethyl)thiomethyl]pyridine.

(h) 2-[(2-aminoethyl)thiomethyl]thiazole.

(i) 3-[(2-aminoethyl)thiomethyl]isothiazole.

(j) 5-amino-2-[(2-aminoethyl)thiomethyl]-1,3,4-thiadiazole.

(k) 3-[(2-aminoethyl)thiomethyl]-1,2,4-triazole.

(l) 4-[(3-(2-aminomethyl)thiopropyl]imidazole.

for 4-methyl-5-((2-aminoethyl)thiomethyl)imidazole in the general procedure of Example 9(i) followed by treatment of the product according to the general procedure of Examples 9(ii) results in the formation of the following S-methyl isothioureas.

a. S-methyl-N,N'-bis-[2-(4-imidazolyl methylthio)ethyl]isothiourea.

b. S-methyl-N,N'-bis-[2-(5-bromo-4-imidazolyl methylthio)ethyl]isothiourea.

c. S-Methyl-N,N'-bis-[2-(1-methyl-2-imidazolyl methylthio)ethyl]isothiourea.

d. S-Methyl-N,N'-bis-[2-(2-imidazolyl methylthio)ethyl]isothiourea.

e. S-Methyl-N,N'-bis-[2-(3-chloro-2-pyridyl methylthio)ethyl]isothiourea.

f. S-Methyl-N,N'-bis[2-(3-bromo-2-pyridyl methylthio)ethyl]isothiourea.

g. S-Methyl-N,N'-bis-[2-(3-methyl-2-pyridyl methylthio)ethyl]isothiourea.

h. S-Methyl-N,N'-bis-[2-(2-thiazolyl methylthio)ethyl]isothiourea.

i. S-Methyl-N,N'-bis-[2-(3-isothiazolyl methylthio)ethyl]isothiourea.

j. S-Methyl-N,N'-bis-[2-(5-amino-2-(1,3,4-thiadiazolyl)methylthio)ethyl]isothiourea.

k. S-Methyl-N,N'-bis-[2-(3-(1,2,4-triazolyl)methylthio)ethyl]isothiourea.

l. S-Methyl-N,N'-bis-[3-(4-imidazolylmethylthio)propyl]isothiourea.

and treatment of these S-methyl isothioureas with methylamine according to the general procedure of Example 10 results in the formation of the following N"-methylguanidines:

a. N,N'-bis-[2-(4-imidazolyl methylthio)ethyl]-N"-methylguanidine.

b. N,N'-bis-[2-(5-bromo-4-imidazolyl methylthio)ethyl]-N"-methylguanidine.

c. N,N'-bis-[2-(1-methyl-2-imidazolyl methylthio)ethyl]-N"-methylguanidine.

d. N,N'-bis-[2-(2-imidazolyl methylthio)ethyl]-N"-methylguanidine.

e. N,N'-bis-[2-(3-chloro-2-pyridyl methylthio)ethyl]-N"-methylguanidine.

f. N,N'-bis-[2-(3-bromo-2-pyridyl methylthio)ethyl]-N"-methylguanidine.

g. N,N'-bis-[2-(3-methyl-2-pyridyl methylthio)ethyl]-N"-methylguanidine.

h. N,N'-bis-[2-(2-thiazolyl methylthio)ethyl]-N"-methylguanidine.

i. N,N'-bis-[2-(3-isothiazolyl methylthio)ethyl]-N"-methylguanidine.

j. N,N'-bis-[2-(5-amino-2-(1,3,4-thiadiazolyl)methylthio)ethyl]-N"-methylguanidine.

k. N,N'-bis-[2-(3-(1,2,4-triazolyl)methylthio)ethyl]-N"-methylguanidine.

l. N,N'-bis-[3-(4-imidazolylmethylthio)propyl]-N"-methylguanidine.

EXAMPLE 15

    ______________________________________                                         Ingredients              Amounts                                               ______________________________________                                         N,N'-bis-[2-(5-methyl-4-imidazolyl-                                            methylthio)ethyl]N"-methylguanidine                                                                     150    mg                                             trihydrochloride.                                                              Sucrose                  75     mg                                             Starch                   25     mg                                             Talc                     5      mg                                             Stearic Acid             2      mg                                             ______________________________________                                    

The ingredients are screened, mixed and filled into a hard gelatin capsule.

EXAMPLE 16

    ______________________________________                                         Ingredients              Amounts                                               ______________________________________                                         N,N'-bis-[2-(5-methyl-4-imidazolyl-                                            methylthio)ethyl]-N"-methylguanidine                                           trihydrochloride.        200    mg.                                            Lactose                  100    mg.                                            ______________________________________                                    

The ingredients are screened, mixed and filled into a hard gelatin capsule. 

What is claimed is:
 1. A compound of the formula: ##STR8## wherein R₁ represents a grouping of the structure

    Het--CH.sub.2 --S--(CH.sub.2).sub.n --

wherein Het is a nitrogen containing 5 or 6 membered heterocyclic ring selected from pyridine and 1,2,4-triazole which is optionally monosubstituted by lower alkyl, hydroxyl, halogen or amino; n is 2 or 3; R₂ is NHR₅ ; R₃ is R₁ ; and R₅ is lower alkyl, or a pharmaceutically acceptable acid addition salt thereof.
 2. A compound according to claim 1 wherein Het is pyridine which is optionally monosubstituted by lower alkyl, hydroxyl, halogen or amino.
 3. A compound according to claim 1 wherein n is
 2. 4. A compound according to claim 1 wherein Het is pyridine optionally monosubstituted by methyl, hydroxyl or halogen.
 5. A compound according to claim 1 wherein R₃ is identical to R₁.
 6. A pharmaceutical composition to block histamine H₂ -receptors comprising, in an effective amount to block said receptors, a compound of claim 1 in combination with a pharmaceutically acceptable diluent or carrier.
 7. A method of blocking histamine H₂ -receptors which comprises administering to an animal in need thereof, in an effective amount to block said receptors, a compound of claim
 1. 